The present invention relates to a light-emitting device using electroluminescence (EL).
Semiconductor lasers have been used as a light source for optical communications systems. Semiconductor lasers excel in wavelength selectivity and can emit light with a single mode. However, it is difficult to fabricate the semiconductor lasers because many stages of crystal growth are required. Moreover, types of light-emitting materials used for semiconductor lasers are limited. Therefore, semiconductor lasers cannot emit light with various wavelengths.
Conventional EL light-emitting devices which emit light with a broad spectral width have been used in some application such as for displays. However, EL light-emitting devices are unsuitable for optical communications and the like, in which light with a narrow spectral width is required.
An object of the present invention is to provide a light-emitting device which can emit light having a remarkably narrow spectral width in comparison with conventional EL light-emitting devices and exhibiting directivity, and can be applied not only to displays but also to optical communications and the like.
First Light-emitting Device
A first light-emitting device according to the present invention comprises a substrate and a light-emitting device section,
wherein the light-emitting device section comprises:
a light-emitting layer capable of emitting light by electroluminescence;
a pair of electrode layers for applying an electric field to the light-emitting layer;
a light-propagation section for propagating light emitted in the light-emitting layer;
an insulation layer disposed between the pair of electrode layers, having an opening formed in part, and capable of functioning as a current concentrating layer for specifying a region through which current to be supplied to the light-emitting layer flows through the opening; and
an optical section for light propagated through the light-propagation section,
wherein the optical section forms photonic band gaps in one dimension or two dimension and has a defect section which is set so that an energy level caused by defects is within a specific emission spectrum, and
wherein the light emitted in the light-emitting layer is emitted with spontaneous emission in one dimension or two dimension inhibited by the photonic band gap.
According to this light-emitting device, electrons and holes are injected into the light-emitting layer respectively from the pair of electrode layers (cathode and anode). Light is emitted when the molecules return to the ground state from the excited state by allowing the electrons and holes to reconnect in the light-emitting layer. At this time, light with a wavelength in the photonic band gap cannot be propagated through the optical section. Only light with a wavelength equivalent to the energy level caused by the defects is propagated through the optical section. Therefore, light with a very narrow emission spectrum width with spontaneous emission inhibited in one dimension or two dimensions can be obtained with high efficiency by specifying the width of the energy level caused by the defects.
In the present invention, the light-propagation section is part of the light-emitting device section and supplies light obtained in the light-emitting layer in the light-emitting device section to a waveguide section. The light-propagation section includes at least the optical section and a member (one of the electrode layers, for example) which is connected with a core layer in the waveguide section. This is also applicable to a second light-emitting device as described later.
According to the first light-emitting device, since the insulation layer functions as a current concentrating layer in the light-emitting device section, the region through which current supplied to the light-emitting layer flows can be specified. Therefore, current intensity and current distribution can be controlled in the region from which it is desired to emit light, whereby light is emitted with high emission efficiency.
In the case where the insulation layer functions as cladding, assuming that the waveguide formed of a light-emitting layer as a core and an insulation layer as cladding, the guide mode of light propagated toward the waveguide section through the light-propagation section can be controlled by specifying the opening of the insulation layer. Specifically, the guide mode of light propagated through the light-emitting layer (core) can be set at a specified value by specifying the width of the region in which light is confined (width perpendicular to the direction in which light is transmitted) by the insulation layer (cladding) The relation between the guide mode and the waveguide is generally represented by the following equation.
Nmax+1xe2x89xa7K0xc2x7axc2x7(n12xe2x88x92n22)xc2xd/(xcfx80/2)
where:
K0: 2xcfx80/xcex,
a: half width of waveguide core,
n1: refractive index of waveguide core,
n2: refractive index of waveguide cladding, and
Nmax: maximum value of possible guide mode.
Therefore, if the parameters of the above equation such as the refractive indices of the core and cladding have been specified, the width of the light-emitting layer (core) specified by the width of the opening of the current concentrating layer may be selected depending on the desired guidemode. Specifically, the width (2a) of the light-emitting layer corresponding to the core in a desired guide mode can be calculated from the above equation by substituting the refractive indices of the light-emitting layer provided inside the current concentrating layer and the insulation layer (current concentrating layer) for the refractive indices of the core and cladding of the waveguide, respectively. It is appropriate to determine the suitable width of the core layer of the waveguide section to which light is supplied from the light-emitting device section while taking into consideration the resulting width of the light-emitting layer, calculated value obtained from the above equation based on the desired guide mode, and the like. Light with a desired mode is propagated from the light-emitting device section toward the waveguide section with high combination efficiency by appropriately specifying the width of the light-emitting layer, width of the core layer, and the like. In the light-emitting device section, the light-emitting layer in the current concentrating layer formed using the insulation layer may not uniformly emit light. Therefore, it is appropriate that the designed values for each member such as the light-emitting layer, light-propagation section, and waveguide section be suitably adjusted based on the width (2a) of the core (light-emitting layer) determined using the above equation so that each member exhibits high combination efficiency.
The guide mode of the light-emitting device is preferably 0 to 1000. In particular, when used for communications, the guide mode is preferably about 0 to 10. Light with a specific guide mode can be efficiently obtained by specifying the guide mode of light in the light-emitting layer in this manner.
As described above, according to the present invention, a light-emitting device which can emit light having a remarkably narrow spectral width in comparison with conventional EL light-emitting devices and exhibiting directivity, and can be applied not only to displays but also to optical communications and the like can be provided.
Second Light-emitting Device
A second light-emitting device according to the present invention comprises a light-emitting device section, and a waveguide section which transmits light from the light-emitting device section, the light-emitting device section and the waveguide section being integrally formed on a substrate,
wherein the light-emitting device section comprises:
a light-emitting layer capable of emitting light by electroluminescence;
a pair of electrode layers for applying an electric field to the light-emitting layer;
a light-propagation section for propagating light emitted in the light-emitting layer;
an insulation layer which is disposed in contact with the light-propagation section and is capable of functioning as a cladding layer; and
an optical section for light propagated through the light-propagation section,
wherein the waveguide section comprises:
a core layer integrally formed with at least part of the light-propagation section; and
a cladding layer integrally formed with the insulation layer,
wherein the optical section forms photonic band gaps in one dimension or two dimension and has a defect section which is set so that an energy level caused by defects is within a specific emission spectrum, and
wherein the light emitted in the light-emitting layer is emitted with spontaneous emission in one dimension or two dimension inhibited by the photonic band gap.
According to the second light-emitting device, at least part of the light-propagation section in the light-emitting device section is continuously and integrally formed with the core layer in the waveguide section, and the insulation layer (cladding layer) in the light-emitting device section is continuously and integrally formed with the cladding layer in the waveguide section. Therefore, the light-emitting device section and the waveguide section are optically connected with high combination efficiency, thereby enabling efficient light propagation.
In this configuration, as the material for the insulation layer, a material which functions as a cladding layer for the light-propagation section is selected. According to this light-emitting device, since at least part of the light-propagation section in the light-emitting device section and the core layer in the waveguide section can be formed and patterned in the same step, fabrication can be simplified. The insulation layer (cladding layer) in the light-emitting device section and the cladding layer in the waveguide section can be formed and patterned in the same step. This also simplifies the fabrication.
According to the present invention, a light-emitting device which has photonic band gaps in one dimension or two dimensions, can emit light with a remarkably narrow spectral width in comparison with conventional EL light-emitting devices and exhibiting directivity, and can be applied not only to displays but also to optical communications and the like can be provided in the same manner as the first light-emitting device.
The opening formed in the insulation layer which functions as a current concentrating layer and a cladding layer is suitably in the shape of a slit extending in the periodic direction of the optical section, specifically, in the direction in which light is waveguided. At least part of the light-emitting layer is suitably formed in the opening formed in the insulation layer. According to this configuration, the region of the light-emitting layer to which current is supplied and the region specified by the current concentrating layer can be self-alignably positioned.
In the first and second light-emitting devices of the present invention, the optical section has a periodic refractive index distribution in one dimension or two dimensions and forms photonic band gaps in one dimension or two dimensions. The optical section may have a structure such as a grating-shaped structure, a multilayer film structure, a columnar or other columnar-shaped structure, or a combination of these structures. Appropriate examples of the optical section are as follows.
(A) The optical section may have a periodic refractive index distribution in a first direction and include a first medium layer and a second medium layer alternately arranged. In the case of using such an optical section, the light-emitting device according to the present invention suitably has a second photonic band gap capable of inhibiting spontaneous emission of light in two dimensions in combination with a first photonic band gap in one dimension formed by the optical section. Light with a very narrow emission spectrum width with spontaneous emission inhibited in two dimensions can be obtained with high efficiency by the optical section and the second photonic band gap.
(B) The optical section may have a periodic refractive index distribution in first and second directions and include a first medium layer arranged in the shape of a tetragonal lattice and a second medium layer. Since photonic band gaps with spontaneous emission inhibited in two dimensions in two directions can be formed by the optical section, light with a very narrow emission spectrum width can be obtained with high efficiency.
(C) The optical section may have a periodic refractive index distribution in first, second, and third directions in one plane and include a columnar first medium layer arranged in the shape of a triangular lattice or a honey-comb lattice, for example, and a second medium layer. Since photonic band gaps with spontaneous emission inhibited in two dimensions in three directions can be formed by the optical section, light with a very narrow emission spectrum width can be obtained with high efficiency.
The light-emitting layer preferably includes an organic light-emitting material as the light-emitting material. Use of organic light-emitting material widens selection of materials in comparison with the case of using a semiconductor material or inorganic material, for example, thereby enabling light with various wavelengths to be emitted.
These light-emitting device may have various embodiments. Typical embodiments are given below.
(a) In a light-emitting device according to a first embodiment, the light-emitting device section may comprise:
a transparent anode formed on the substrate and capable of functioning as at least part of the light-propagation section;
the optical section formed in part of the anode;
the insulation layer having an opening facing the optical section;
the light-emitting layer, at least part of which is formed in the opening in the insulation layer; and
a cathode.
(b) In a light-emitting device according to a second embodiment, the light-emitting device section may comprise:
an intermediate substrate disposed on the substrate, the optical section being formed in part of the intermediate substrate;
a transparent anode which is formed on the optical section in the intermediate substrate and is capable of functioning as at least part of the light-propagation section;
the insulation layer having an opening facing the anode;
the light-emitting layer, at least part of which is formed in the opening in the insulation layer; and
a cathode.
Some of the materials which can be used for each section of the light-emitting device according to the present invention are illustrated below. These materials are only part of the conventional materials. Materials other than these materials may also be used.
Light-emitting Layer
The material for the light-emitting layer is selected from conventional compounds in order to obtain light with a predetermined wavelength. Any of organic and inorganic compounds may be used as the materials for the light-emitting layer. It is preferable to use organic compounds from the viewpoint of wide variety of compounds and film-formability.
As examples of such organic compounds, aromatic diamine derivatives (TPD), oxydiazole derivatives (PBD), oxydiazole dimers (OXD-8), distyrylarylene derivatives (DSA), beryllium-benzoquinolinol complex (Bebq), triphenylamine derivatives (MTDATA), rubrene, quinacridone, triazole derivatives, polyphenylene, polyalkylfluorene, polyalkylthiophene, azomethine zinc complex, polyphyrin zinc complex, benzooxazole zinc complex, and phenanthroline europium complex which are disclosed in Japanese Patent Application Laid-open No. 10-153967, and the like can be given.
Moreover, as the materials for the organic light-emitting layer, conventional compounds disclosed in Japanese Patent Application Laid-open No. 63-70257, No. 63-175860, No. 2-135361, No. 2-135359, No. 3-152184, No. 8-248276, No. 10-153967, and the like can be used. These compounds may be used either individually or in combination of two or more.
As examples of inorganic compounds, ZnS:Mn (red region), ZnS:TbOF (green region), SrS:Cu, SrS:Ag, SrS:Ce (blue region), and the like can be given.
Optical waveguide
The optical waveguide includes a layer which functions as a core, and a layer which has a refractive index lower than that of the core and functions as cladding. Specifically, these layers include the light-propagation section (core) and the insulation layer (cladding) in the light-emitting device section, the core layer and the cladding layer in the waveguide section, substrate (cladding), and the like. Conventional inorganic and organic materials may be used for forming the layers which form the optical waveguide.
As typical examples of inorganic materials, TiO2, TiO2xe2x80x94SiO2 mixture, ZnO, Nb2O5, Si3N4, Ta2O5, HfO2, and ZrO2 disclosed in Japanese Patent Application Laid-open No. 5-273427, and the like can be given.
As typical examples of organic materials, conventional resins such as thermoplastic resins, thermosetting resins, and photocurable resins can be given. These resins are appropriately selected depending on the method of forming the layers and the like. For example, use of a resin cured by energy of at least one of heat or light enables utilization of commonly used exposure devices, baking ovens, hot plates, and the like.
As examples of such materials, a UV-curable resin disclosed in Japanese Patent Application No. 10-279439 applied by the applicant of the present invention can be given. As UV-curable resins, acrylic resins are preferable. UV-curable acrylic resins having excellent transparency and capable of curing in a short period of time can be obtained by using commercially-available resins and photosensitizers.
As specific examples of basic components of such UV curable acrylic resins, prepolymers, oligomers, and monomers can be given.
As examples of prepolymers or oligomers, acrylates such as epoxy acrylates, urethane acrylates, polyester acrylates, polyether acrylates, and spiroacetal-type acrylates, methacrylates such as epoxy methacrylates, urethane methacrylates, polyester methacrylates, and polyether methacrylates, and the like can be used.
Examples of monomers include mono functional monomers such as 2-ethylhexyl acrylate, 2-ethylhexyl methacrylate, 2-hydroxyethyl acrylate, 2-hydroxyethyl methacrylate, N-vinyl-2-pyrrolidone, carbitol acrylate, tetrahydrofurfuryl acrylate, isobornyl acrylate, dicyclopentenyl acrylate, and 1,3-butanediol acrylate, bifunctional monomers such as 1,6-hexanediol diacrylate, 1,6-hexanediol dimethacrylate, neopentyl glycol diacrylate, neopentyl glycol dimethacrylate, ethylene glycol diacrylate, polyethylene glycol diacrylate, and pentaerythritol diacrylate, and polyfunctional monomers such as trimethylolpropane triacrylate, trimethylolpropane trimethacrylate, pentaerythritol triacrylate, and dipentaerythritol hexaacrylate.
These inorganic and organic materials are illustrated taking only light confinement into consideration. In the case where the light-emitting device section has a light-emitting layer, hole transport layer, electron transport layer, and electrode layer, and at least one of these layers functions as the core or cladding layer, the materials for these layers may be employed as the material for the layers of the optical waveguide.
Hole Transport Layer
When using an organic light-emitting layer in the light-emitting device section, a hole transport layer may be formed between the electrode layer (anode) and the organic light-emitting layer, as required. As the materials for the hole transport layer, materials conventionally used as hole injection materials for photoconductive materials or materials used for a hole injection layer of organic light-emitting devices can be selectively used. As the materials for the hole transport layer, any of organic and inorganic substances having a function of either hole injection or electron barrier characteristics may be used. As specific examples of such substances, substances disclosed in Japanese Patent Application Laid-open No. 8-248276 can be given.
Electron Transport Layer
In the case of using an organic light-emitting layer in the light-emitting device section, an electron transport layer may be formed between the electrode layer (cathode) and the organic light-emitting layer, as required. Materials for the electron transport layer are only required to have a function of transporting electrons injected from the cathode to the organic light-emitting layer. Such materials can be selected from conventional substances. For example, a substance disclosed in Japanese Patent Application Laid-open No. 8-248276 can be given as specific examples.
Electrode Layer
As the cathode, electron injectable metals, alloys, electrically conductive compounds with a small work function (for example, 4 eV or less), or mixtures thereof can be used. Materials disclosed in Japanese Patent Application Laid-open No. 8-248276 can be given as specific examples of such electrode substances.
As the anode, metals, alloys, electrically conductive compounds with a large work function (for example, 4 eV or more) or mixtures thereof can be used. In the case of using optically transparent materials as the anode, transparent conductive materials such as CuI, ITO, SnO2, and ZnO can be used. In the case where transparency is not necessary, metals such as gold can be used.
The optical section can be formed by conventional methods without specific limitations. Typical examples of such methods are given below.
(1) Lithographic Method
A positive or negative resist is irradiated with ultraviolet rays, X-rays, or the like. The resist layer is patterned by development to form an optical section. As a patterning technology using a resist formed of polymethylmethacrylate or a novolak resin, technologies disclosed in Japanese Patent Applications Laid-open No. 6-224115 and No. 7-20637 can be given.
As a technology of patterning polyimide by photolithography, for example, technologies disclosed in Japanese Patent Applications Laid-open No. 7-181689 and No. 1-221741 can be given. Furthermore, Japanese Patent Application Laid-open No. 10-59743 discloses a technology of forming an optical section of polymethylmethacrylate or titanium oxide on a glass substrate utilizing laser ablation.
(2) Formation of Refractive Index distribution by Irradiation
The optical waveguide section of the optical waveguide is irradiated with light having a wavelength which produces changes in the refractive index to periodically form areas having a different refractive indices on the optical waveguide section, thereby forming an optical section. As such a method, it is preferable to form an optical section by forming a layer of polymers or polymer precursors and polymerizing part of the polymer layer by irradiation or the like to periodically form areas having a different refractive index. Such a technology is disclosed in Japanese Patent Applications Laid-open No. 9-311238, No. 9-178901, No. 8-15506, No. 5-297202, No. 5-32523, No. 5-39480, No. 9-211728, No. 10-26702, No. 10-8300, and No. 2-51101, and the like.
(3) Stamping Method
An optical section is formed by, for example, hot stamping using a thermoplastic resin (Japanese Patent Application Laid-open No. 6-201907), stamping using an UV curable resin (Japanese Patent Application Laid-open No. 10-279439), or stamping using an electron-beam curable resin (Japanese Patent Application Laid-open No. 7-235075).
(4) Etching Method
A thin film is selectively patterned using lithography and etching technologies to form an optical section.
The methods for forming the optical section are described above. In summary, the optical section formed of at least two areas, each having a different refractive index, and can be fabricated, for example, by a method of forming these two areas from two materials having a different refractive index, a method of forming the two areas from one material and modifying the material forming one of the two areas so that the two areas have a different refractive index, and the like.
Each layer of the light-emitting device can be formed by a conventional method. For example, each layer of the light-emitting device is formed using a suitable film-forming method depending on the materials therefor. As specific examples of such a method, a vapor deposition method, spin coating method, LB method, ink jet method, and the like can be can be given.
FIG. 1 is an oblique view schematically showing a light-emitting device according to a first embodiment of the present invention.
FIG. 2 is a plan view schematically showing the light-emitting device according to the first embodiment of the present invention.
FIG. 3 is a cross-sectional view along the line X1xe2x80x94X1 shown in FIG. 2.
FIG. 4 is a cross-sectional view along the line X2xe2x80x94X2 shown in FIG. 2.
FIG. 5 is a cross-sectional view along the line X3xe2x80x94X3 shown in FIG. 2.
FIG. 6 is a cross-sectional view along the line Yxe2x80x94Y shown in FIG. 2.
FIG. 7A is a plan view showing a fabrication step of the light-emitting device according to the first embodiment of the present invention, and FIGS. 7B to 7D are cross-sectional views along the line Axe2x80x94A, line Bxe2x80x94B, and line Cxe2x80x94C shown in FIG. 7A, respectively.
FIG. 8A is a plan view showing a fabrication step of the light-emitting device according to the first embodiment of the present invention, and FIGS. 8B to 8D are cross-sectional views along the line Axe2x80x94A, line Bxe2x80x94B, and line Cxe2x80x94C shown in FIG. 8A, respectively.
FIG. 9A is a plan view showing a fabrication step of the light-emitting device according to the first embodiment of the present invention, and FIGS. 9B to 9D are cross-sectional views along the line Axe2x80x94A, line Bxe2x80x94B, and line Cxe2x80x94C shown in FIG. 9A, respectively.
FIG. 10A is a plan view showing the fabrication step of the light-emitting device according to the first embodiment of the present invention, and FIGS. 10B to 10D are cross-sectional views along the line Axe2x80x94A, line Bxe2x80x94B, and line Cxe2x80x94C shown in FIG. 10A, respectively.
FIG. 11A is a plan view showing the fabrication step of the light-emitting device according to the first embodiment of the present invention, and FIGS. 11B and 11C are cross-sectional views along the line Bxe2x80x94B and the line Cxe2x80x94C shown in FIG. 11A, respectively.
FIG. 12A is a plan view showing the fabrication step of the light-emitting device according to the first embodiment of the present invention, and FIG. 12B is a cross-sectional view along the line Bxe2x80x94B shown in FIG. 12A.
FIG. 13A is a plan view showing the fabrication step of the light-emitting device according to the first embodiment of the present invention, and FIG. 13B is a cross-sectional view along the line Bxe2x80x94B shown in FIG. 13A.
FIG. 14A is a plan view showing the fabrication step of the light-emitting device according to the first embodiment of the present invention, and FIGS. 14B and 14C are cross-sectional views along the line Bxe2x80x94B and the line Cxe2x80x94C shown in FIG. 14A, respectively.
FIG. 15 is a cross-sectional view schematically showing a light-emitting device according to a second embodiment of the present invention.
FIG. 16 is a cross-sectional view schematically showing a light-emitting device according to a third embodiment of the present invention.
FIG. 17 is an enlarged cross-sectional view showing a section indicated by the symbol A in FIG. 16.
FIG. 18 is a plan view showing an optical section according to the third embodiment of the present invention.
FIG. 19 is a cross-sectional view schematically showing a light-emitting device according to a fourth embodiment of the present invention.
FIG. 20 is an enlarged cross-sectional view showing a section indicated by the symbol B in FIG. 19.
FIG. 21 is a plan view showing an optical section according to the fourth embodiment of the present invention.
FIG. 22 is a view showing a modification example of the optical section.
FIGS. 23A and 23B are views showing further modification examples of the optical section.